The present invention relates to process fluid pressure measurement devices. In particular, the present invention relates to a process seal for a process fluid pressure measurement system.
Process devices, such as process fluid pressure transmitters, generally sense pressure using a pressure sensor coupled to at least one isolation diaphragm. The isolation diaphragm isolates the pressure sensor from process fluids that are being sensed. Process fluids, which can be highly corrosive, are thus kept isolated from the pressure sensor to avoid corrosion or damage to the pressure sensor. Pressure is transferred from the isolation diaphragm to the pressure sensor, which has a sensing diaphragm that deflects in response to the applied pressure. The pressure is transferred from the isolation diaphragm to the sensing diaphragm using a substantially incompressible isolation fluid in a passageway that fluidically couples the isolation diaphragm to the sensing diaphragm. U.S. Pat. No. 4,833,922 entitled MODULAR PRESSURE TRANSMITTER and U.S. Pat. No. 5,094,109 entitled PRESSURE TRANSMITTER WITH STRESS ISOLATION DEPRESSION show pressure transmitters of this type.
Various corrosion resistant, high cost metals are sometimes used for the isolation diaphragms. Tantalum, for example, is a material that is very resistant to corrosion, but has a considerably higher melting point than other materials that are conventionally used for isolation diaphragms, such as 316L stainless steel, Hastalloy C and Monel. The isolator housing mounting the isolation diaphragm is generally constructed of a stainless steel alloy which will generally have much lower melting point than a material such as tantalum.
The utilization of materials such as tantalum for applications that require extremely high corrosion resistance creates difficulties with respect to the manufacture of such products. Specifically, since tantalum has a melting point that is substantially higher than the other metals used in the process device, traditional manufacturing methods such as welding are sometimes impractical for joining tantalum to a much lower melting-point metal such as stainless steel. Further, the mixed-metal weld is not able to meet some requirements of NACE (National Association of Corrosion Engineers) for the application. Thus, there are metallurgical incompatibilities between the corrosion resistant material and the rest of the metal used for the process device. As used herein, metallurgically incompatible means that the two materials cannot be welded together practically, or will create an unacceptable mixed-metal weld. Further, the cost of the tantalum or other suitable high-corrosion resistant metal generally drives design criteria that utilize as little of the material as possible. Further still, the manufacturing difficulties encountered in incorporating the tantalum isolation diaphragm into the process device are generally manifested in the form of higher overall product costs and longer product lead time.